1. Field of the Invention
This invention relates to a sodium-sulfur electric cell and a process for manufacturing the same. More particularly, the present invention is concerned with a sodium-sulfur electric cell having excellent strength and airtightness and a process for manufacturing the same.
2. Description of the Prior Art
In conventional processes for manufacturing a sodium-sulfur electric cell as described in U.S. Pat. No. 4,037,027, attention was mainly directed to the corrosion resistance of anode and cathode tubes, and a stainless steel material was used for the anode and cathode tubes. However, no study was made on the adoption of materials having a low coefficient of thermal expansion for the purpose of attaining a high strength, not to mention a method of joining these materials with each other to attain excellent airtightness.
Specifically, although the above-mentioned prior art method uses a stainless steel as the cathode and anode tubes mainly taking into consideration the corrosion resistance to sodium, sulfur and sodium polysulfide, no attention was directed to an improvement in the strength of the joints or to the airtightness of the joints. Therefore, the prior art method was accompanied by problems such as low joint strength and insufficient airtightness.
The sodium-sulfur electric cell made of stainless steel according to the prior art method is primarily intended for the adaptation to small-sized products (e.g., electric cells having a capacity of 80 W/hr and an outer diameter of 30 mm.phi.). When this method is applied to large-sized products (e.g., electric cells having a capacity of 600 W/hr or more and an outer diameter of 75 mm.phi.), an anode tube made of stainless steel is joined to a cathode tube made of stainless steel through the medium of an aluminum insert. However, this causes a problem that the larger the outer diameter, the lower the joint strength of the joints. It is noted in this connection that the aluminum insert is used solely because of excellent corrosion resistance thereof to sodium and sulfur.